1. Field of the Invention
The field of art disclosed herein pertains generally relates to a device and method for detecting substances on a surface.
2. Description of the Related Art
A need exists for a method and a device that is quickly and quantitatively sensitive to the properties of the surface of a material or an object. One reason for making these measurements is to determine if a surface is properly prepared and ready for further processing, such as printing, bonding, or sealing. Another reason is to determine if a cleaning process is working correctly to remove contaminants from a surface. Another reason is to provide a feedback signal that can be used to control a process such as a coating process or a corona, flame, or plasma treatment process.
One technique for measuring the properties of a surface involves observing the behavior of drop of liquid placed onto the surface. When a drop of liquid is deposited onto a surface and allowed to come to equilibrium, it forms a shape determined by the balance of several forces: the liquid surface tension, the solid surface energy, the gravitational force on the liquid, and the strength of attraction between the liquid and the surface on which it rests. The shape of the drop is defined by the contact angle θ. This is the angle between the surface and a tangent to the drop at the point of intersection of the drop with the surface. A small contact angle is observed when the liquid spreads on the surface, while a large contact angle is observed when the liquid beads on the surface. More specifically, a contact angle less than 90° indicates that wetting of the surface is favorable, and the fluid will spread over a large area on the surface; while contact angles greater than 90° generally means that wetting of the surface is unfavorable so the fluid will minimize its contact with the surface and form a compact liquid droplet.
FIG. 1 illustrates a diagram of a small liquid drop on a surface. The dashed line represents a circle with the same radius of curvature R as the spherical drop. Angle θ is the contact angle, d is the diameter of the contact patch, h is the maximum height of the drop above the surface, and A is the maximum cross-sectional area of the drop.
The relationship of the contact angle to the surface tension of the liquid and the substrate surface energy is defined by the Young equation:γs=γsl+γl cos θ  (1)Where γs=substrate surface energy
γsl=substrate-liquid interfacial energy
γs=liquid surface tension
Contact angles are usually measured using a device known as a contact angle goniometer. A drop of the probe liquid is placed on the surface to be interrogated, the plane of the surface is brought into the line of sight of a telescope containing a measuring scale, and a reticle in the telescope is made tangent to the drop profile at the point of contact with the surface. The angle that this line makes with the surface is defined as the contact angle. In one or more embodiments, the contact angle can also be determined by computerized analysis of a digital image of the drop.
A contact angle does not have to be directly measured. Small drops (typically less than about 10 microliters) form a spherical shape. The contact angles in these cases can be calculated from other geometrical features of the drop, such as the diameter of the contact patch, the radius of curvature, the height, or the cross sectional area, and the volume. For larger drops that are no longer spherical, the contact angle can be approximated quite precisely through the use of more complex equations.
Generally, if the water contact angle is smaller than 90°, the solid surface is considered hydrophilic and if the water contact angle is larger than 90°, the solid surface is considered hydrophobic. Many polymers exhibit hydrophobic surfaces. Highly hydrophobic surfaces made of low surface energy (e.g. fluorinated) materials may have water contact angles as high as ˜120°. Some materials with highly rough surfaces may have a water contact angle even greater than 150°, due to the presence of air pockets under the liquid drop. These are called superhydrophobic surfaces. If the liquid molecules are strongly attracted to the solid molecules then the liquid drop will completely spread out on the solid surface, corresponding to a contact angle of 0°. This is often the case for water on bare metallic or ceramic surfaces.
Because the contact angle is determined by the interaction of the liquid with the uppermost few molecular layers of the surface, the presence of an oxide layer or of some other substance on the solid surface can significantly change the contact angle. Hydrophobic substances tend to increase the contact angle of water, while hydrophilic substances tend to decrease the water contact angle. Less than a monomolecular layer of some substances can change the contact angle by several degrees, a readily measurable amount. This makes contact angle measurements especially useful for detecting the presence of contaminants or other substances on a surface. Furthermore, one can establish a quantitative relationship between the amount of a substance on a surface and the contact angle established with a particular liquid, making contact angle measurements useful for quantifying the amount of substance on a surface in some instances.
The use of contact angle measurements to confirm the existence of a certain surface composition can be hampered by the presence of substances on the surface that can interact with the liquid, for example through dissolution or chemical reaction. This interaction will change the properties of the surface and the liquid and can affect the contact angle. An example is the presence of a hydrophilic substance such as a surfactant on a metal or ceramic surface. When drop of a liquid such as water is placed on a surface that has surfactant on it, the surfactant will tend to dissolve into the water. This removes the surfactant from the surface, increasing its surface energy, while simultaneously decreasing the surface tension of the water. The combination results in a low contact angle. This low contact angle can be confused with the low contact angle presented by a clean metal or ceramic surface. Because of the possibility of confusing a clean surface with a surface having a soluble or reactive substance on it, contact angle measurements have not been widely used for confirming surface properties. Consequently, a need exists for an improved system and method of detecting a contact angle that accounts for the presence of soluble or reactive substances on the surface.